Couple of things... Get Yuki spayed. I can't understand why she isn't done already.
Yuki and Tigger health checked very thoroughly. Tigger in particular. It is possible that Yuki is attacking Tigger because she is sensing Tigger is sick. I would suggest getting her checked asap.
You were away for a week. There is no telling what happened while you were gone. Cats are very sensitive to change. Any change to their routine can lead to a large amount of stress. It is possible even a very loud clap of thunder would be enough to stress her out. Her human was gone so there was no one there to go to for comfort. It's possible that Tigger was close to her when something stressful happened and now she associates Tigger to that stress.
Try the sites that SCM gave you to check out on reintroducing.
And please, before you decide to just give up on Yuki, let us know. Cats don't just attack a long time mate without reason.